In ISO 7816 standards for IC cards provided with an external terminal, a processing device having an external terminal complying with ISO 7816-2 can perform not only data transfer complying with ISO 7816-3 but also USB (Universal Serial Bus) transfer complying with ISO 7816-12. Here, in ISO 7816-12 specifications, an external terminal device does not supply a clock to the processing device. Therefore, when performing USB transfer complying with ISO 7816-12, it is required to operate the processing device by the use of a clock generated in the processing device in principle.
Also, in the data transfer complying with ISO 7816-3, specifications about a clock used in the data transfer between an external terminal device and a processing device are defined, but specifications about a clock used in the processing device are not defined. When a clock generated in the processing device is used, since it is required to operate modules such as an oscillator and a PLL, power consumption increases in comparison with the case of the operation using only a clock supplied from an external terminal device. Therefore, a clock supplied from an external terminal device is used for the operation in some cases.